The alkyl monoesters of fatty acids from vegetable oils and animal fats, known as biodiesel, have been proposed as an alternative, non-toxic, biodegradable and renewable diesel fuel. Many studies have shown that the properties of biodiesel are very close to diesel fuel. Therefore, biodiesel fuel can be used in diesel engines with little or no modification. Biodiesel has a higher cetane number than diesel fuel, no aromatics, no sulfur, and contains 10 to 11% oxygen by weight. These characteristics of biodiesel reduce the emission of carbon monoxide, hydrocarbons, and particulate matter in the exhaust gas compared to diesel fuel.
Refined vegetable oils are the main starting materials for producing biodiesel (1,2). Methanol is added into vegetable oils to transesterify triglycerides into fatty acid methyl esters (FAME). Most commonly, a base catalyst such as methoxide is used to catalyze the transesterification reaction. However, refined vegetable oils are expensive. Cheaper starting materials are desirable for reducing the cost of biodiesel products.
One source of cheaper starting material is the soapstock generated during vegetable oil refining process. The soap is of limited value and some soapstock processors acidify it to convert the soap to free fatty acids (FFA) and also to release the entrained neutral oil. The oil layer composed of oil and a high level of FFA, often referred to as acid oil, is usually sold as a feed ingredient. Generally, soapstock is produced at 6% of the volume of crude oil produced (3), accounting for 1.8 billion pounds in the U.S. annually. Therefore, there is great potential to create a value-added biodiesel product from such inexpensive material. Other sources of cheaper starting materials include waste vegetable oils from restaurants and rendered animal fats. Approximately 2.5 billion pounds of waste restaurant fats are collected from restaurants and fast-food establishments in the U.S. As the acid oil from soapstock, the restaurant waste oils and fats also contain high levels of FAA.
The base catalyst-catalyzed transesterification reaction, commonly used for generating FAMEs from refined vegetable oils, cannot be used to produce biodiesel from the cheaper starting materials with high FFA content identified above since the FAAs in the materials form soaps with the base catalyst. One way people have tried to solve the problem was to convert FFAs in acid oil into methyl esters by using a large excess amount of methanol and an acid catalyst followed by the commonly used base catalyst-catalyzed transesterification reaction to convert oil to FAMEs. The large amount of methanol used is for driving the reaction to completion. Due to the large amount of methanol used in this process, methanol must be recycled to reduce the cost. However, the recycling process itself is expensive.